The goal of this project has been to characterize monocyte-macrophage responses to extracellular microorganisms as they relate to protective immunity. Using the extracellular protozoan Giardia lamblia as a prototype extracellular organism, we are investigating a) the mechanisms of monocyte chemoattration and cytotoxicity for G. lamblia; b) the ability of lymphocytes to regulate monocyte cytotoxicity of G. lamblia; and c) the effect of cytomegalovirus infection on monocyte responses to G. lamblia.